1. Field of the Invention
The invention relates to a gene for improving aroma production in a plant, and more particularly to a gene, protein, protein complex and method for improving aroma production in a plant.
2. Description of the Related Art
Flowers are unique organs that advertise their attractiveness in form, color and fragrance for insects, birds and mammals to assure pollination. Orchidaceae is one of the largest monocotyledon families, containing more than 25,000 species. In orchids, large quantities of pollen in masses are spread by animals (bees, moths, flies and birds), and the floral scents serve as attractants for species-specific pollinators. These pollinators play an important role in orchid floral diversification advantageous to the evolution of an obviously successful family. However, the biochemistry of fragrance production and the mechanisms regulating its emission in orchids remains sketchy.
To date, no simple, efficient, and reliable culture methods for scented orchids have been developed. Although the sympatric speciation of orchids is linked to differences in their floral odors, the large genome size, long life cycle and regeneration time and inefficient transformation system render the orchid scent biology difficult to explore. Furthermore, several scent and scentless species are cross-incompatible, which leads to the difficulty of producing scented offspring via traditional breeding. In some successful cases of cross-breeding, the offspring have diluted scent or even totally expel the scent-producing ability. Thus, to increase crop quality, the molecular breeding of scent species by introducing key enzymes regulating scent production to species/cultivars already with good characters is of interest.
Terpenoids belong to a large family of plant secondary metabolites, and their corresponding alcohols possess useful properties such as fragrance, flavor, insecticidal properties and characteristics that make them useful as pharmaceutical agents. In addition, primary metabolites like abscisic acid, carotenoids, chlorophyll, gibberellins, quinine electron carriers and steroids are also terpene-derived (van Schie et al., 2007, Plant J, 52, 752-762). Monoterpens formed from geranyl diphosphate (GDP, C10, is synthesized from dimethylallyl diphosphate (DMADP, C5) and isopentenyl diphosphate (IDP, C5) by GDP synthase (GDPS). IDP and DMADP are used by prenyltransferases to catalyze the synthesis of the general terpene backbones. GDPS is a member of the short-chain trans-prenyltransferase family, and it also includes farnesyl diphosphate synthase (FDPS) and geranylgeranyl diphosphate synthase (GGDPS), which synthesize farnesyl diphosphate (FDP, C15) and geranylgeranyl diphosphate (GGDP, C20), respectively (Ogura and Koyama, 1998, Chem. Rev., 98, 1263-1276; Reiling et al., 2004, Biotechnol Bioeng, 87, 200-212). These enzymes provide the acyclic branch-point intermediates for isoprenoid biosynthesis and control the flux into various terpenoid products. However, regulation and sharing of precursor pools are only starting to be explored.
The distribution of GDPS appears to be limited within nature. It has been described in the plants Mentha piperita (Burke et al., 1999, Proc Natl Acad Sci USA, 96, 13062-13067), Arabidopsis thaliana (Bouvier et al., 2000, Plant J, 24, 241-252), Abies grandis (Burke and Croteau, 2002, Arch Biochem Biophys, 405, 130-136), Antirrhinum majus, Clarkia breweri (Tholl et al., 2004, Plant Cell, 16, 977-992), Lycopersicon esculentum (van Schie et al., 2007, Plant J, 52, 752-762) and Phalaenopsis bellina (Hsiao et al., 2008, Plant J, 55, 719-733). GDPSs are either homomeric or heteromeric; the A. grandis, Arabidopsis and tomato GDPS, which contain the Asp-rich motifs and presume function as homodimers (Bohlmann et al., 2000, Arch Biochem Biophys, 375, 261-269; Burke and Croteau, 2002, Arch Biochem Biophys, 405, 130-136; van Schie et al., 2007, Plant J, 52, 752-762). The M. piperita, A. majus, and C. breweri GDPSs comprise heterodimers of a small (Mentha_SSU, Antirrhinum_SSU and Clarkia_SSU) and a large subunit (Mentha_LSU and Antirrhinum_LSU), which share only 22-38% identity with homomeric angiosperm GDPSs or GGDPSs (Burke et al., 2004, Arch Biochem Biophys, 422, 52-60; Tholl et al., 2004, Plant Cell, 16, 977-992). Notably, the small subunit from these plants lacks the Asp-rich motifs and is inactive per se (Tholl et al., 2004, Plant Cell, 16, 977-992). It appears to control the length of the chain synthesized by the catalytic large subunit, because interaction of the small subunit with GGDPS results in the conversion of functional GGDPS into GDPS (Tholl et al., 2004, Plant Cell, 16, 977-992). Meanwhile, the GDPS large subunit shares high amino acid sequence identity with GGDPS from plants (50%-75%), but the Mentha_LSU and Antirrhinum_LSU per se forms an active GGDPS enzyme that produces GGDP (Tholl et al., 2004, Plant Cell, 16, 977-992). Thus, the frequency of occurrence of the GDPS large subunit in plants has remained an open question.
Previously, floral scents in P. bellina (Orchidaceae, monocot) are demonstrated to be rich in the monoterpenes, geraniol and linalool and their derivatives (Hsiao et al., 2006, BMC Plant Biol, 6, 14). Identification a dual-function GDPS that lacks the Asp-rich motifs normally required for scent production but contains instead a glutamate-rich (Glu-rich) motif and is able to form a homodimer. Recent researches showed that GDPS small subunit from M. piperita, A. majus, and C. breweri is capable of modifying the chain length specificity of its catalytic partner and can bind to a variety of bona fide GDPS_LSU and GGPPS enzymes (Burke and Croteau, 2002, J Biol Chem, 277, 3141-3149; Tholl et al., 2004, Plant Cell, 16, 977-992). None of the previous studies of heterodimer GDPS have been performed in plant of monocots. It is curious that whether or not the heterodimer GDPS exists in orchid flower.